In June 1812 virtually all of continental Europe was under Napoleon's control and his invasion of Russia was an attempt to force Tsar Alexander I to submit to the terms of an unfavourable treaty that Napoleon had imposed upon him four years earlier. Having gathered nearly half a million soldiers from France as well as all of the vassal states of Europe, Napoleon entered Russia at the head of the largest army assembled up to that moment in European history. By September of that year, Napoleon had advanced to the gates of Moscow, where he found the city deserted and bereft of supplies. The great fire of Moscow broke out leaving the French troops without shelter in Winter, and Napoleon had no choice but to order his troops to march back home. Given the harsh winter conditions and lack of food the retreat home was particularly difficult and as a result only ten thousand men survived. The campaign ensured Napoleon's downfall and Russia's status as a leading power in post-Napoleonic Europe.